1. Field of the Invention
The present invention relates generally to gas turbine engines and, more specifically, to an aircraft gas turbine engine having improved compressor stall recovery capabilities.
2. Description of the Related Art
A typical aircraft jet engine includes a compressor, a combustor, and a turbine. The compressor is rated according to its pressure ratio and mass airflow. As with any airfoil device, effectiveness gradually increases up to an optimum point followed by a rapid decay known as stall or surge. In the design of high performance engines, there is usually a fine line between optimum performance and stall.
Each stage of a compressor increases the pressure until the air leaves the compressor at a significantly higher pressure than it had upon entering the compressor. If compressor rpm is constant and compressor airflow is decreased, the airflow vector becomes shorter and the angle of attack is increased. If the airflow is constant and the rpm is increased, the angle of attach will be increased. An airfoil stalls and lift is lost when maximum design angle of attack is exceeded. One or more compressor blades and possibly the entire compressor could stall under such conditions.
To increase the stall margin when operating under conditions other than the compressor design speed, the inlet guide vanes and multiple stages of the stator vanes are made variable. The angular position and the resultant air deflection are a function of compressor inlet temperature (CIT) and engine speed. Vane angular position is controlled automatically through the main fuel control or engine control system.
Generally speaking, if the air within the compressor reaches an unstable condition, a stall might result. A stall condition can be sensed and indicated by the engine control system, or the manifestations of the condition can be observed, such as a rapid exhaust gas temperature (EGT) increase fluctuation, rpm fluctuation, engine pressure ratio (EPR) decrease or fluctuation, vibration caused by compressor pulsation and poor engine response to throttle movements. More severed compressor stalls can cause very loud noises and may be accompanied by flame, vapor or smoke at the engine exhaust and/or inlet.
Compressor stalls generally result in potentially damaging increases in EGT. Since excessive temperatures shorten the life of engine parts, it is desirable to implement stall recoveries as soon after detection is possible.
One recovery technique is to use compressor bleed valves to relieve air from the compressor. Bleeding of the compressor decreases the airflow across the rear stages and increases the flow across the forward stages. This deters choking of the rear stages and stalling of the forward stages.
Compressor bleed may avoid stalling but the penalty for doing so is a loss of thrust in that the effective pressure ratio of the compressor during the time the valves are open is reduced. U.S. Pat. No. 4,186,556 describes a technique of increasing the stall margin by monitoring the amount of air that is bled off from a compressor and then biasing the acceleration schedule in accordance with the amount of bleed air.
Typically, a combustor will have a plurality of fuel nozzles disposed at equidistantly spaced intervals around the annular combustor. The nozzles are interconnected to a manifold which is fed with fuel from a fuel line connected to a fuel pump.
Another technique for implementing stall recovery is to cut back the supply of fuel to the combustor section, either by use of the throttle or an intervening flow control device independent of the throttle. For example, in U.S. Pat. No. 4,768,338, a surge condition recovery technique includes cutting back fuel to the combustor upon detection of a surge. Stall recovery results from the removal of back pressure as a result of less fuel consumption. The back pressure is uniformly distributed around the annularly shaped combustor. Typically, a combustor will have a plurality of fuel nozzles disposed at equidistantly spaced intervals around the annular combustor. The nozzles are interconnected to a manifold which is fed with fuel from a fuel line connected to a fuel pump.
Another example of fuel flow rate control is found in U.S. Pat. No. 4,722,180, wherein the fuel delivery rate is controlled by a valve disposed in the fuel line between the manifold and the pump.
With a system that relies on either a reduction in fuel delivery to the combustor, or an increase in the amount of air bled from the combustor, a concomitant reduction in thrust will result. Thus, a need exists for a compressor stall recovery method and apparatus which does not rely on a total or partial reduction in fuel flow, to effect a compression stall recovery.